Conventionally, focusing control has been performed by detecting a defocus amount between the position along the optical axis of the focal plane on which the image of a photographic subject is focused and the position along the optical axis of the film surface, converting this defocus amount into an amount by which a focus lens should be moved (a focus lens moving amount), and driving a dedicated focus lens by just that moving amount.
In addition, in an image sensing apparatus equipped with an electrical image sensor, an arrangement has been proposed in which focusing control is performed by detecting the predetermined spatial frequency component of the sensed image and driving the focus lens so that that value peaks.
At present, in order to make cameras more compact, there is a growing need to make the lens more compact as well. In order to make the lens more compact, it is necessary to move and to stop the focus lens with a high degree of accuracy.
Conventionally, controlling a gross focusing control lens and a fine focusing control lens so as to perform high-accuracy focusing control has been proposed (see Japanese Patent Laid-Open No. 1-158882).
In addition, controlling the gross focusing control lens and the image sensors for the fine focusing control (capable of moving along the optical axis) to perform high-accuracy focusing control also has been proposed (Japanese Patent Laid-Open No. 2002-350714).
Where an effort is made to improve the accuracy of focusing control in the typical conventional auto focus camera lens control apparatus described above, one possible method of doing so, for example, involves enhancing the detection and analysis capabilities of an encoder that monitors the amount of movement of the focus lens so as to improve focus lens positioning accuracy. In such an instance, in an effort to improve the detection and analysis capabilities of the encoder, the detection capability of the encoder is itself enhanced, or detection is undertaken after expansion and conversion of the focus lens linear movement amount. However, this sort of method requires micro machining technology, mechanical vibrations can no longer be ignored, and so forth, giving rise to self-imposed limits on improvements in accuracy.
In order to further facilitate an understanding of the problems of the conventional art, a description will now be given of the conventional zoom lens barrel focusing control using FIGS. 25 and 26.
First, a description is given of FIG. 25, at the top of which is shown schematically the arrangement of lens groups in the zoom lens barrel and an image sensor in a wide-angle end state, beneath which is shown the arrangement of the lens groups in the zoom lens barrel and the image sensor in a telephoto-end state. Three graphs are shown at the bottom of the drawing, in which the vertical axis for all three graphs is elapsed time T. The graph on the left shows the lens position on the horizontal axis, showing the track of the movement of a second lens group 1004 and of a third lens group 1006 positioned closer to an image sensor 1008 during focusing control. The graph in the center shows focus evaluation values calculated by a signal processing circuit based on the image signals obtained by the image sensor 1008 on the horizontal axis, whose maximal value indicates an in-focus state. The graph on the right shows focusing sensitivity on the horizontal axis. Focusing sensitivity is a ratio of the amount of movement of the focal plane in the direction of the optical axis to the amount of movement of the moving lens group. For example, a focusing sensitivity of −8.5 in which only the third lens group 1006 moves indicates that, when the third lens group 1006 moves 1 μm, the focal plane moves 8.5 μm in the opposite direction.
It is assumed that the lens groups are positioned at the telephoto end and focused on infinity, with the photographic subject to be sensed positioned 2 m away from the camera.
As shown in the left graph in FIG. 25, from a time T0, the third lens group 1006, which is a concave lens system, begins to move linearly toward the image sensor 1008. The focusing sensitivity at this time is −8.5. When a third lens group stepping motor, not shown, moves one step, the third lens group 1006 is driven 12.5 μm toward the image sensor 1008, and, as shown in FIG. 26, the position of the focal plane changes from P0 to P1 and the focus evaluation value increases from V0 to V1. When the third lens group 1006 is driven one step further toward the image sensor 1008, the position of the focal plane changes from P1 to P2. When the third lens group 1006 is driven one step further still toward the image sensor 1008, the position of the focal plane changes from P2 to P3. In FIG. 25, the focus evaluation value reaches its maximal value (maximum focus evaluation value) VP at a time T1, after which it begins to decline. The maximum focus evaluation value VP is stored.
Using a time T2 as the starting point for the lens position, the third lens group stepping motor is rotated in reverse, driving the third lens group 1006 in reverse. At a time T3, the position of the focal plane returns to position P2 (=P4) and the focus evaluation value returns to V2.
If 90 percent of the maximum focus evaluation value VP is considered a pass level value VS, and if the focus evaluation value V is at or above VS, then it is possible to sense a fully focused photograph. In FIG. 26, PF is the position of the focal plane where the maximum focus evaluation value VP is obtained, and δt is the range of positions of the focal plane at which, at the telephoto end, the focus evaluation value is at or above the pass level value VS.
In the conventional example described in FIGS. 25 and 26, in order to raise the focus evaluation value V to or above the pass level value VS, the focal plane must move between focal plane positions P2 and P3. However, the third lens group stepping motor enables the focal plane to move to positions P2 and P3 but cannot stop between the two positions, and therefore, at the telephoto end, the position of the focal plane cannot be made to be within the range δt, making it impossible to sense a fully focused photograph.
In addition, in the focusing control apparatus described in Japanese Patent Laid-Open No. 1-158882, it is necessary to provide stepping motors or other drive means for both the gross focusing control lens and the fine focusing control lens, leading to an increase in the size of the apparatus and moreover posing high technological hurdles in terms of switching between gross and fine control.
In addition, in the camera system described in Japanese Patent Laid-Open No. 2002-350714, in order to obtain high-accuracy focusing, an image sensor drive apparatus inside the camera body is required separate from and in addition to the lens drive apparatus unit inside the lens barrel, which leads to an increase in the overall size of the camera.